The claim from Sir Bernard Hogan-Howe, the Metropolitan Police Commissioner, that millions of workers should take random tests is likely to prove controversial with civil liberties groups and trade unions. In a speech to the All-Party Parliamentary Group on Cannabis and Children, Sir Bernard said anyone who failed the mandatory tests and refused help to get clean should lose their jobs.

He said testing should take place in "all occupations" but cited Britain's public sector workers in particular, claiming the fear of losing jobs would act as a deterrent to cannabis use.

"It seems to me we have got to plant in people's minds something to affect the demand as well as supply," Sir Bernard said. "You can think of many occupations where if you were working with a colleague you would want to be sure in fact that they were drug-free."

He said parents born in the 1960s and 70s, were failing to warn their children about the dangers of modern marijuana, including skunk. He said the potency of cannabis had increased "five-fold" in the last half century.

"It is refreshing to hear a senior police office who is deeply aware of the dangers of today's cannabis," said Mary Brett, from the Cannabis Skunk Sense charity.

In the US, random drug testing is commonplace. Companies say the tests help identify theft risk and improve employee reliability and productivity.

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